Geology in space

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Winter in the northern hemisphere is synonymous with snow (at least in films and Victorian novels). The Earth is unusual because it has water can commonly be found on its surface in solid, liquid and gas form, and both solid and liquid water can rain/snow/sleet/drizzle from the sky. So what happens on other bodies in the solar system?

Clouds of ice crystals on Mars – Ready to snow? (NASA/JPL)

Most water on the surface of Mars is now trapped in its glaciers/ice caps, but what about snow? Whilst there is less water vapour in the Martian atmosphere than the Earth’s, Mars still has clouds made of ice crystals. These ice crystals can fall slowly out of the sky, but at night under the right conditions, snowstorms can occur. Most of this snow will vaporise before it hits the ground, and any that do make it to the ground will likely vaporise during the day.

Maxwell Montes the highest mountain on Venus, appears brighter than the surroundings because of sulphide frost (Image NASA/JPL)

Venus has the hottest surface temperature of any planet in the solar system. Water snow is not possible. The atmosphere of Venus contains lots of sulphur and temperatures hot enough to melt lead, it is so thick we can’t see the surface of the planet so we have collected radar data to understand what it looks like. One feature of this data is that on the tops of some high mountains there is an increase in the number of radio waves reflected, giving the appearance that the tops of mountains are brighter, just as snow on the top of mountains is brighter than the surrounding rock.

Whilst it is far too hot for snow on Venus, it is thought that these mountain tops are covered in a form of frost caused by metal sulphides, which forms only at high elevations and it’s this which makes is appear more reflective to radar.

Mercury, my main planet of study, doesn’t have snow-capped peaks, being too hot over the majority of the planet and no atmosphere. So the nearest feature to snow is ice in the craters close to the north and south poles, these craters are permanently in shadow protecting the ice from the boiling Sun. Without an atmosphere there isn’t any form of precipitation other than a daily micrometeorite shower.

The south pole of Mercury, the black areas are permanently in shadow and thought to contain water ice. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Titan, a moon of Saturn is much to far out from the Sun to have water snow, and any water near the surface, just like in the bleak midwinter, is like a stone. It does snow on Titan, near the poles hydrocarbons can crystallize out in the atmosphere and then fall as snow down onto the surface.

Another moon of Saturn, Enceladus, is probably the most visually spectacular example, geysers on the south pole can be seen erupting water vapour into space. Some of this water is lost in orbit and forming the E ring of Saturn, but some of the water falls back down onto the surface as ice crystals. As this fall back it is happening without an atmosphere it doesn’t really count as snow but is a great excuse to show the stunning photos of the plumes.

Old river sediments in Brazil and the Central African Republic contain minerals found nowhere else on Earth; a form of diamond, dark black or green-gray, mixed with a lot of graphite (the stuff inside pencils) and often with small bubbles inside.

These diamonds are unusual for a variety of reasons, they make up some of the largest diamonds ever found and yet the origin of them is unknown. Most diamonds come from kimberlites; volcanic tubes which shot diamonds up from the Earth’s mantle up to near the surface (in some areas diamonds are mined from kimberlites, in others, they are found in rivers downstream of them). Carbonado diamonds don’t have any signs of kimberlites associated with them they also don’t include any material from the Earth’s mantle, which can often be found trapped inside diamonds.

Diamond is formed at high pressure and temperatures and so doesn’t often have any pores or bubbles within them, but these types of diamonds are often very porous (full of holes). The type of carbon which makes them up is also unusual. Whilst all diamonds are made of carbon, carbonados are made of a lighter form (isotope) of carbon than other diamonds found on Earth. Dating has shown them to be at least 3.8 billion years old, half a billion years older than the oldest known kimberlites adding to the mystery of their source.

Their two geographical locations can be easily explained by plate tectonics, whilst currently, separate the two were at one stage part of the same continent. (Later geological processes affecting more the countries to the west of CAR in Africa). This point source location, unusual isotopic composition, and the high-pressure high-temperature but also high gas pressure (allowing all the bubbles to form). Narrow down the possibilities of how these diamonds formed and suggest an extraterrestrial origin.

The remnants of a supernova – any diamonds here? (NASA/ESA/JHU/R.Sankrit & W.Blair)

The most exciting hypothesis is that these diamonds formed in a supernova; the exploding death of a star, this would explain both the carbon isotopes and also the bubbles. So whilst Thor’s hammer might have been forged from a neutron star, these diamonds found on Earth may come from the remains of an exploding star. Other theories include being knocked off a white dwarf star or coming from one of the giant planets in the solar system or even from an exoplanet. This is an area of active research and further understanding of the processes going on in these high-pressure temperature environments will help narrow down the candidates but it’s amazing to think that these diamonds may have come from outside our solar system.

There are a lot of upcoming space missions to be excited about; BepiColombo’s trip to Mercury, ExoMars, the Europa Clipper and New Horizons second flyby to name a few. One that has me really excited is Psyche.

Psyche is a NASA mission to explore the asteroid Psyche 16. This is an M-Class asteroid, which means that it is made dominantly of metals, mostly Nickle and Iron, compared most others types of asteroids which contain much more silicates (rock) within them. This unusual composition suggests that it is a relic left over from the formation of the planets.

Early in the life of the solar system, a protoplanetary disk of dust began to form clumps of rock as they collided, gradually these lumps of rocks grew into objects known as planetesimals.

When these planetesimals became large enough, the internal heat, generated by a mixture of gravitational energy, radioactive decay, and heat generated by collisions allowed those larger than about 30 km to internally separate out like oil and water; the denser materials sank to the bottom whilst the lighter material floated to the top. Iron, nickel, and a range of other metals sank to the bottom, it is this same process which formed the Earth’s core.

We have never sampled the Earth’s core, only being able to use seismic, magnetic, gravity and density data to infer information about it, as well as studying the limited number of meteorites which may be remnants of a core exposed by collisions with other planetesimals.

Psyche is the only known exposed planetesimal core in the solar system. This makes it a truly unique and exciting place to visit as it will enable us to look directly at and take measurements on the same kind of material which makes the core of the rocky planets. This will be the first time we have visited a metal body and is likely to look unlike any other body is so far seen in the solar system, possibly with impact craters showing metallic jagged edges.

Psyche above Psych SSL/Peter Rubin

The Psyche mission is scheduled to launch in 2022 and reach the asteroid in 2026, it will be exciting to see what new insights about the formation of the solar system and our own core.

Since Galileo first pointed a telescope at the Moon and saw its marked surface people have been debating how the craters they observed formed, for much of history there were two interpretations; either an impact crater or formed by volcanic eruptions.

Impact or Volcano Picard crater -Luna Oribter 4 (NASA)

It was not until the space age when close up photos (and later moon landings) revealed most of the creators on the surface were formed by impacts (volcanoes do exist on the moon but form very different features). But what are the differences and how can you tell remotely without landing?

Barringer Crater Arizona, formed by a meteorite impact (USGS)

On first glance, both types of craters can look very similar, both would form round features with rims. Both can have ejecta around them, formed from the material thrown out from the inside (either the material the surface is made from or ash from volcanic eruptions).

Aniakchak, Alaska volcanic caldera (US National Parks service)

However when you think about how they formed you can understand some of the differences when looking at them: volcanoes are explosive, expansions of material often with repeated events close to or in the same place.

Most volcanoes are built upon the landscape and so often the inside of the crater will be at a higher altitude than the surrounding terrain (with the exception of Maars due to interactions with water under the ground and features called calderas which are collapsed volcanoes). Whereas impact craters are formed by a surface beeing excavated by an impactor, whilst the sides of the crater can be built up from material thrown out of the crater floor itself will be lower than the surrounding terrain.

Volcanoes often show evidence of their volcanic nature around them, whether it is lava or ash flows inside and on the sides of the volcano. The volcano itself will be built up of volcanic rock. An impact craters wall will be made up of the same material excavated from the site. there are of course exceptions to this, for example on Mercury some of the impact craters are filled in with lava which broke through the bottom of the crater, however, the wall of the crater is of different material to later lavas which fill in the basin.

Volcanic craters can also be asymmetrically distorted by the wind or the direction of the eruption (or deformed by later eruption events). The overwhelming majority of impact craters are circular. Larger craters also have a central mound formed by the rebounding material which you don’t see in volcanoes (though new volcanic cones can grow in existing craters).

There are lots more features of craters evident when you get a close-up view on the ground but they are hard to observe from satellites where most of our planetary data comes from. As with much of geology, by observing a combination of these features a story of where a crater came from and how it formed can be built up from remote sensing data.

Previous posts have looked at the nature of tectonics on the terrestrial planets and how they are very different to Earth. Further out in the solar system, it is not the planets which are of geological interest but the moons, many of these are ice worlds. Cores of rock covered in layers of ice. Some of these have pockets or layers of water in them generated by tidal friction generating heat.

Evidence for this layer of water or at least an icy slush is shown by fountains of ice sprayed into space on both Enceladus and Europa showing that these moons are active places however, there are other telltale signs of a system similar to plate tectonics.

Photos of Europa, an Ice moon of Jupiter, revealed a cracked surface covered in bands. Limited cratering suggests that surface is relatively young (around 40-90 million years). Some of the surface shows repeating bands similar to spreading ridges seen on Earth. Where the surface ice cracks and moves apart and new water rises up and fills the gap with ice.

Europa as seen by Galileo (Nasa/JPL-Caltech/SETI institute)

These same spreading ridges and associated bands are also clearly seen on Miranda, a moon of Uranus, which has alternating patterns around a central rift, just like is seen on the ocean plates of Earth.

Where tectonics must differ is what happens at the other end; ice is very buoyant making it difficult to find a form of subduction which would allow pulling of the crust down into the water/ slush layers like the plates on Earth. As the surfaces of these moons cannot be ever increasing in diameter, something has to be happening to the ice.

On Europa there no regional scale mountain belts where the crust thickens to accumulate this extra crust. Instead, there is some evidence of complex fault systems. Similar to subduction zones, but which behave differently and have been termed subsumption zones – one section of the ice crust is forced under another, where it rapidly becomes incorporated into the underlying water ice… so rather than continuing to sink like the plates on Earth it rapidly becomes part of the underlying layers, enabling tectonics to continue.

Miranda as seen my Voyager 2 (Nasa/JPL)

The ice moons of the outer solar system are harsh alien places, but tectonically they may be the most Earth-like of all the bodies in the solar system.

No body is perfect; forces on both the inside and outside of them push and pull them around. This leads to cracks and wrinkles on surfaces which represent the passage of time. On planetary bodies, this is referred to as tectonics.

Most bodies have tectonic features spread out over their surface. Earth is different. The surface is divided up into interlocking blocks which move around over time, called tectonic plates. These plates are constantly moving: colliding, separating or moving past each other, this movement causes the majority (but not all) of earthquakes and volcanoes on the Earth. The vast majority of tectonic features are related to either current or historical plate boundaries.

The Earth’s tectonic plates, courtesy of the U.S. Geological Survey

The driving force behind plate tectonics is dominated by the gradual overturning of the mantle as it moves heat from the core to the crust in a process called convection. The plates are also pulled by old cold crust sinking down and pulling the plate with it.

These forces linked to the moving of plates transfer stresses through the crust and lead to the rocks deforming. On a large scale, this leads to several features on the Earth’s surface which can be seen from space. In future posts, we’ll look at some of the features on other planets but first, let’s have a look at a few features formed by plate tectonics on the Earth:

Faults

A fault offsets banded rock south of the Tien Shan mountains as seen by Landsat 8 (USGS/NASA)

Faults are breaks in rocks along which movement occurs and are associated with earthquakes. They release strain built up in the crust and are found on all plate margins, including those that slide past each other like the San Andras fault. These are often seen as breaks and offsets in geological patterns.

Rifts

In areas where the crust is under tension it can be pulled apart, the rocks break into a series of faults, which form a staircase-like feature dropping down to form a valley. As the crust gets thinner as it is pulled apart it enables volcanism to start and volcanoes form along the valley (such as in the East African Rift valley).

This rifting can continue and the crust can break into two separate plates. with a volcanic ridge in the middle which generates new crust as the two sides move apart. These features form the seas and the oceans.

East African rift valley, formed by multiple faults (seen as lines going diagonally across the image) which lead to low lying areas as the plate thins (NASA/JPL)

Mountains

Mt Everest and other mountains as seen from STS-066 (NASA, JSC)

In areas where plates collide with each other, the outcomes vary depending on the type of crust colliding. If it involves a thinner denser crust called oceanic crust (which is the type generated by mid-ocean ridges) subduction occurs – an oceanic plate will be forced down and sink into the mantle. This activity leads to a trough and then a line of volcanoes behind them. Where two pieces of thicker, less dense continental crust collide the parts crash into each other they crumple rather thank sink and form mountain ranges such as the Himalayas.

Earth is unique with its highly active plate tectonic system defining its surface, other planets have been and are tectonically active but in different ways with features spread out and controlled by different tectonic systems.